The present invention relates to a projection television system comprising projection television display tubes having curved faceplates. One or more of the tubes have interference fitters on the inside surfaces of their respective faceplates.
Existing projection television systems are regarded as having a moderate brightness on the projection screen; a poor chromaticity due to the commonly used Tb activated green phosphors, because the contribution of the orange and red spectral lines is too large for these phosphors; a loss in resolution on the projection screen due to chromatic aberration of the lenses, especially for the commonly used green (Tb-activated) and blue (ZnS:Ag) phosphors; and a moderate contrast.
In order to mitigate these problems it has been proposed for example in European Patent Publication No. 0170320 to provide interference filters between the phosphor layer and the glass faceplate. These interference filters transmit light of a desired wavelength in forward and near forward directions. At larger angles, up to 90 degrees to the faceplate normal, the interference filter reflects the light of this wavelength. This light is rescattered in the phosphor layer and does have a chance to leave the tube at a small angle to the faceplate normal, resulting in a gain in brightness in the near forward directions. At smaller wavelengths the filters transmit light up to larger angles, so that the relative gain in brightness in the forward direction becomes lower. At longer wavelengths the filter transmits the light up to smaller angles, or even blocks the light in forward direction. As the filter works color selectively, the chromaticity improves and the chromatic aberration due to projection lenses reduces.
At large focal length lenses, that is those where the focal length is greater than the diagonal of the scanned phosphor area on the faceplate, for example 130 mm for a 5" (125 mm) diagonal scanned area and 180 mm for a 7" (175 mm) diagonal scanned area, the entrance pupil of the lens is relatively far away from the phosphor layer, and therefore the acceptance angles of the lens are relatively small, even in the corner of the faceplate. However, if one uses a closed rear projector, there is a trend to use smaller focal length lenses (down to half of the diagonal of the tube faceplate), in order to keep the cabinet size acceptable. The entrance pupil of these lenses is closer to the faceplate, which results in an increase of the lens acceptance angles for light from the corners of the faceplate. Using the same type interference filter as has been used previously with the larger focal length lenses results in:
(a) An unacceptable drop in brightness going from the center to the corner of the projection screen, because the interference filter reflects the light of the desired wavelength at these large angles. This drop in brightness comes on top of the normal drop in brightness due to the obliquity of the principal ray with respect to the faceplate normal and due to vignetting by the lens elements.
(b) A color shift over the projection screen, because the filters work color selectively. A shift to the shorter wavelengths (blue) will occur on the screen, going from the center to the corners.
One way of mitigating these problems is to use an interference filter which transmits light up to larger angles. However, the gain in brightness is then much smaller, and a color shift will still occur. It has been suggested in unpublished British patent Application No. 8513558 to which U.S. Pat. No. 4,683,398 corresponds, to use a projection TV tube with an interference filter and a curved faceplate. In this way the angles to the faceplate normal tilt over to match the lens acceptance angles. The smallest radius of faceplate curvature to be practical is expected to be about 2.5 to 3 times the diagonal of the scanned phosphor area on the faceplate. At smaller radii problems occur with dynamic focusing of the electron beam, filter deposition, phosphor deposition and corrections in the deflection of the electron beam due to picture distortion. This means that with the small focal length lenses (&lt;0.75 times the diagonal of the scanned phosphor area on the faceplate), which enable systems to be built into the desired small cabinet sizes, the problems due to the too large acceptance angles of the lens still occur.
An examination of the gains in luminance and chromaticity achieved by using short focal length lenses with display tubes having curved faceplates on which are deposited interference filters having dielectric layers with substantially constant thickness over the faceplate area shows that the gains at the center for filters having relatively low cut-off angles is greater than for filters having relatively high cut-off angles whereas the situation was substantially the opposite at the corners. Although there is invariably a lower light output from the corners of the screen compared to the center it is desired to reduce the differential between them.